The present invention relates to particle size measurement devices, and devices used for counting particles in fluid streams wherein the particles are discriminated on the basis of their size.
More specifically, the present invention pertains to a sensing element used in such systems, the sensing element comprising a light source and light sensitive element positioned on opposite sides of a fluid flow channel. Particles carried by a fluid stream in the flow channel cast a shadow on the light sensitive element, which shadow causes a change in the output signal level from the light sensitive device. By utilizing sensitive, well known prior art signal detectors, the change in this signal can be correlated with a given particle size, and thus particles of various sizes can be measured and counted, the detector signal being supplied to a discriminator circuit which divides the particles into size groups.
While the state of the particle size detecting art is quite advanced, numerous problems have continued unsolved in prior art devices. These problems both limit the permissible environment in which the sensors may be utilized and, at the same time, limit the sensor accuracy. It will be understood by those skilled in the art that particle size discriminating devices, such as those of the present invention, are used in a wide variety of applications. For example, these devices are used for monitoring the quantity and size of impurities in liquid streams to determine when liquids meet cleanliness specifications. They are also used in the production of powders and granulated material to monitor grain size and size distribution. Other uses for the equipment are well known and are limited only by the requirement that the particles to be measured must be carried past the light source and sensing elememt in a fluid or air stream and must be small enough to pass through the flow channel provided between these elements.
In the past there has been a requirement for compromise between the smallest particle sizes which could be measured by such devices and the accuracy and reliability of the devices. As smaller and smaller particle sizes are measured, the light flow path is commonly also reduced in cross-sectional area. For a given light sensor sensitivity (and accepting the fact that the detector signal level, and the changes in detector signal level during measurement, must all be above ambient electrical noise levels), there is a requirement that the light source be made as bright as possible. This, in general, has required, in all practical applications, that the light souce be a high intensity filament bulb. Such bulbs, of course, are subject to fatigue and failure in shock or vibration environments and this fact, by itself, has significantly limited the permissible applications for, and the reliability of sensors.
The use of filament sources (which are not point light sources), has significantly reduced the ability of any lens systems to accurately collimate the light source. Therefore, light passing through the flow channel has generally not been a collimated parallel beam of light, but rather a converging or diverging light pattern. Either of these patterns substantially reduces the accuracy of the measurement system, since the ability of a particular sized particle to cast a particular sized shadow on the light sensor is dependent upon the parallel nature of the light beam. A filament source, particularly in a high intensity bulb, has a relatively large physical size, which prohibits accurate collimation.
Prior to the present invention, no satisfactory solution has been found which both permitted highly accurate, highly dependable size measurement of small particles, while at the same time providing a sensor structure which was operable in vibration and shock environments.